hmac only works with `str` type in Python 2

If you provide unicode type variables, it throws a `TypeError`.

Full trace back:

    2014-12-29 03:06 ERROR    task          test BUG: Unhandled error in plugin em
    ail: character mapping must return integer, None or unicode
    Traceback (most recent call last):
      File "/opt/flexget/local/lib/python2.7/site-packages/flexget/task.py", line 439, in __run_plugin
        return method(*args, **kwargs)
      File "/opt/flexget/local/lib/python2.7/site-packages/flexget/event.py", line 22, in __call__
        return self.func(*args, **kwargs)
      File "/opt/flexget/local/lib/python2.7/site-packages/flexget/plugins/output/send_email.py", line 274, in on_task_abort
        self.on_task_output(task, config)
      File "/opt/flexget/local/lib/python2.7/site-packages/flexget/plugins/output/send_email.py", line 270, in on_task_output
        send_email(subject, content, config)
      File "/opt/flexget/local/lib/python2.7/site-packages/flexget/plugins/output/send_email.py", line 110, in send_email
        mailServer.login(config['smtp_username'], config['smtp_password'])
      File "/usr/lib/python2.7/smtplib.py", line 600, in login
        (code, resp) = self.docmd(encode_cram_md5(resp, user, password))
      File "/usr/lib/python2.7/smtplib.py", line 564, in encode_cram_md5
        response = user + " " + hmac.HMAC(password, challenge).hexdigest()
      File "/usr/lib/python2.7/hmac.py", line 75, in __init__
        self.outer.update(key.translate(trans_5C))
    TypeError: character mapping must return integer, None or unicode

See also http://bugs.python.org/issue5285.